Tons of potential causes, some quicker, easier, or lower risk than others to rule out.
Easy candidates: dehydration; low blood sugar; junk food diet; nervous energy; boredom; need socialization; need a change of scenery
Moderate difficulty: insufficient sleep; need more sunlight or fresh air or nature; lack of intellectual challenges; lack of creative outlet; need for exercise; diet that isn’t just junk food but isn’t well balanced or healthy either
Difficult: depression, anxiety, or burnout; longer term or more subtle nutritional deficiencies; loneliness